If you get to a spot in the game where you think you cleared a room and then you fall out of frame forever, that's the end of the game. Congratulations!
I would say that its not rage inducing. Simply restarting the level isn't that bad as they are all super short. This game is pretty fun, though the music and sounds got too annoying after the 3rd level.
In the level select screen the die in the center rolls 1-6 and then adds an enemy into the respective level. This is where the theme comes in. I didn't find a way to make the adding of enemies obvious but it happens.
